bpo-38659: [Enum] add _simple_enum decorator (GH-25285)

add:

_simple_enum decorator to transform a normal class into an enum
_test_simple_enum function to compare
_old_convert_ to enable checking _convert_ generated enums
_simple_enum takes a normal class and converts it into an enum:

@simple_enum(Enum)
class Color:
    RED = 1
    GREEN = 2
    BLUE = 3

_old_convert_ works much like _convert_ does, using the original logic:

# in a test file
import socket, enum
CheckedAddressFamily = enum._old_convert_(
        enum.IntEnum, 'AddressFamily', 'socket',
        lambda C: C.isupper() and C.startswith('AF_'),
        source=_socket,
        )

test_simple_enum takes a traditional enum and a simple enum and
compares the two:

# in the REPL or the same module as Color
class CheckedColor(Enum):
    RED = 1
    GREEN = 2
    BLUE = 3

_test_simple_enum(CheckedColor, Color)

_test_simple_enum(CheckedAddressFamily, socket.AddressFamily)

Any important differences will raise a TypeError

Lib/ast.py

@@ -27,7 +27,7 @@
 import sys
 from _ast import *
 from contextlib import contextmanager, nullcontext
-from enum import IntEnum, auto
+from enum import IntEnum, auto, _simple_enum
 
 
 def parse(source, filename='<unknown>', mode='exec', *,
@@ -636,7 +636,8 @@ class Param(expr_context):
 # We unparse those infinities to INFSTR.
 _INFSTR = "1e" + repr(sys.float_info.max_10_exp + 1)
 
-class _Precedence(IntEnum):
+@_simple_enum(IntEnum)
+class _Precedence:
     """Precedence table that originated from python grammar."""
 
     TUPLE = auto()